This application is related to U.S. patent application Ser. No. 09/502,868, now abandoned titled xe2x80x9cMethod for Producing Devices Having Piezoelectric Films,xe2x80x9d filed concomitantly herewith by inventors Bower, Pastalan, and Rittenhouse and assigned to the present assignee (hereinafter the xe2x80x9cBower applicationxe2x80x9d), which is incorporated herein by reference.
The invention relates to a method for producing electronic devices containing a piezoelectric film comprising use of a rotating magnetron sputtering system. The invention is particularly useful in fabricating acoustic resonators and semiconductor devices.
Communications systems typically include a variety of devices (e.g., filters, mixers, amplifiers, integrated circuits, and so forth). Communications systems are useful for transmitting information (e.g., voice, video, data) relayed by means of wireless links, twisted pair, optical fibers, and so forth. As wireless communications systems become more advanced, signals are being transmitted at higher frequencies (e.g., PCS, ISM, etc). As systems are continually developed in response to market pressures, the demand for increased performance and reduced size intensifies. Market forces demand increased integration and reduction of component size.
Resonators such as Bulk Acoustic Wave (BAW) resonators are important components in the fabrication of bandpass filters and other related semiconductor devices. The BAW resonator is a piezoelectric resonator that essentially comprises a film of piezoelectric material (e.g., a crystalline AlN film), deposited between at least two electrodes. Upon application of voltage to such a structure, the piezoelectric material will vibrate in an allowed vibrational mode at a certain frequency. Piezoelectric resonators are thus useful in discriminating between signals based on frequency diversity (e.g., a bandpass filter), and in providing stable frequency signals (e.g., as in a frequency stabilizing feedback element in an oscillator circuit).
Typically, the performance and resonant frequency of the piezoelectric resonator will depend upon the composition, thickness, and orientation of the piezoelectric material. The resonant frequency of a piezoelectric material is typically inversely proportional to its thickness; thus, for piezoelectric resonators to operate at high frequencies {e.g., frequencies greater than xcx9c700 Megahertz (MHz) up to 10 Gigahertz (10 GHz)}, the thickness of the piezoelectric film must be reduced to a thin film (e.g., having a thickness ranging from about 500 nm to about 10 xcexcm). The coupling between electrical and mechanical energy of a piezoelectric resonator is dependent on the crystalline orientation of the atoms comprising the piezoelectric film. The induced strain (i.e., stress wave) in a piezoelectric film in response to applied voltage (i.e., electric field) can only occur from the advantageous alignment of the crystalline axis within the piezoelectric film. An example of an advantageous film orientation is  less than 002 greater than  of AlN perpendicular to the substrate.
Piezoelectric film quality may be affected by the method used to form the film. Typically, sputter deposition or reactive sputter deposition techniques have been used. Sputter deposition involves a vacuum deposition process in which a sputtering target is bombarded with ions, and the atoms of the target material are mechanically ejected from the target and deposited onto a nearby substrate. In reactive sputtering, a reactive gas is introduced into the deposition chamber and reacts with the target material to produce a film that is sputtered onto the substrate, either directly or upon further reaction with freed target material. In DC reactive sputtering, a direct current electrical potential is applied within the sputtering chamber in which a reactive sputtering process is carried out. However, typical sputtering and reactive sputtering techniques, including DC reactive sputtering, often do not provide adequate deposition rates. A pulse DC sputtering method for efficiently depositing thin films of piezoelectric materials such as aluminum nitride (AlN), e.g., with improved control over the direction and delivery of the reactive gas, is described in U.S. patent application Ser. No. 09/145,323, to Miller et al., xe2x80x9cPulse DC Reactive Sputtering Method for Fabricating Piezoelectric Resonators,xe2x80x9d filed Sep. 1, 1998, assigned to the present assignee and incorporated herein by reference. In Miller et al., the quality of the piezoelectric films is improved with the techniques used to deposit the films, i.e., the pulse width of the positive portion of the applied voltage is adjusted based on its effect on the desired film constituency, stress, and texture.
Magnetron sputtering systems are known in which magnetically-enhanced targets are used to confine the plasma discharge along a particular path and enhance the flow of target material. See, e.g., U.S. Pat. No. 5,830,327 to Kolenkow, xe2x80x9cMethods and Apparatus for Sputtering with RotatingMagnet Sputter Sourcesxe2x80x9d; U.S. Pat. No. 5,693,197 to Lal et al., xe2x80x9cDC Magnetron Sputtering Method and Apparatusxe2x80x9d: and U.S. Pat. No. 5,378,341 to Drehman et at, xe2x80x9cConical Magnetron Sputter Source,xe2x80x9d all of which are incorporated herein. Use of magnetron sputtering has been problematic, however, for depositing silicon dioxide films. Because silicon dioxide is a good insulator, a film sufficiently thick to cause arcing problems is rapidly formed at certain areas of the target, i.e., splats or regions of silicon dioxide may be deposited on the target surface so that it is not uniformly biased, and eventually, the target may become coated to the point where it is no longer conductive and the deposition may stop. Thus, magnetron reactive sputtering has not been conventionally used to deposit quality silicon dioxide films. See, e.g., U.S. Pat. No. 5,683,558 to Sieck et al., xe2x80x9cAnode Structure for Magnetron Sputtering Systems,xe2x80x9d at col. 1, lines 53-55 (xe2x80x9cThe arcing associated with silicon dioxide has prevented planar magnetron reactive sputtering from being efficiently utilized to deposit quality silicon dioxide filmsxe2x80x9d). Additionally, previous methods of depositing insulating films (including piezoelectric films) have involved use of RF sputtering utilizing fixed magnets.
As may be appreciated, those in the field of communications systems and components continue to search for new methods for increasing system performance and integration. In particular, it would be advantageous to provide new methods for improving the quality of piezoelectric films. A high-quality AlN piezoelectric film deposited on a reflecting multi-layer acoustic mirror stack is a method to produce high-quality, RF front-end filters for GHz applications. These objectives and further advantages of this invention may appear more fully upon considering the detailed description given below.
Summarily described, the invention embraces a quality-assurance method for use in the fabrication of piezoelectric films for electronic devices, particularly resonators. The method comprises determining the surface roughness of an insulating layer on which the piezoelectric film is to be deposited and achieving a surface roughness for the insulating layer that is sufficiently low to achieve the high-quality piezoelectric film. According to one aspect of the invention, the low surface roughness for the insulating layer is achieved with use of a rotating magnet magnetron system for improving the uniformity of the deposited layer. According to other aspects of the invention, the high-quality piezoelectric film is assured by optimizing deposition parameters or monitoring and correcting for the surface roughness of the insulating layer pre-fabrication of the piezoelectric film.